Silicon carbide (SIC) is a refractory carbide that decomposes at approximately 2400.degree. C. The material has several crystallographic forms (polytypes) which are stable with an appreciable deficiency of carbon (sub-stoichiometric) and may also contain excess carbon. Often, the SiC properties, e.g., the electrical conductivity, is determined by the carbon content. The hexagonal alpha (.alpha.) and cubic beta (.beta.) crystallographic structures are the most common. SiC is a fair electrical conductor, has very good thermal conductivity, and has excellent physical stability. The material is insoluble in acids; however, it may be dissolved in molten potassium hydroxide (KOH), SiC is oxidation resistant by virtue of its forming a surface layer of SiO.sub.2 in a high temperature oxidizing environment.
SiC is used as a hard and wear resistant coating material and as a high temperature semiconductor material. SiC coatings are used on graphite heaters to prevent sublimation of the graphite and as a protective coating on graphite at high temperatures in oxidizing atmospheres. Since starting materials, i.e., powders, can be obtained in very pure form and fabricated in an impurity-free environment, SiC structures are used in applications such as furnace liners and high temperature fixturing to avoid contaminating semiconductor device materials.
Chemical vapor deposition (CVD) is one method of preparing both cubic and hexagonal SiC coatings if the substrate can withstand the high temperatures necessary to obtain dense, high quality SiC material and the SiC-substrate thermal coefficient of expansion mismatch is not such that excessive stresses develop which cause loss of adhesion. When diffusion occurs across an interface, voids may form at the interface (Kirkendall porosity) which may result in poor adhesion.
The chemical vapor deposition technique is rather limited in the size of the parts that can be coated since they need to be contained in a high temperature furnace. For large or numerous parts, a "hot-wall" CVD reactor is used. Complex geometrical shapes can be coated since the deposition is not "line-of-sight". However, flow variations over the surface can give varying coating thickness and material properties. Generally, CVD is a relatively expensive coating process since the deposition rate is slow.
A preferred method of silicon powder deposition is, therefore, the vacuum plasma spray (VPS) method, since the VPS process alleviates ambient thermochemistry effects by controlled spraying in a reduced-pressure inert gas. Vacuum plasma spraying is a versatile and rapid method of applying high purity, high performance protective coatings. Typically, VPS involves plasma spraying at 20-100 mbar.